[Possible consequences of subclinical hypothyroidism].

Subclinical hypothyroidism is defined as elevated TSH in the presence of normal free T4 and T3 levels. This review discusses the following questions concerning subclinical hypothyroidism that have not been solved yet: 1) does elevated TSH always mean failure of the thyroid gland? 2) Do patients with subclinical hypothyroidism always develop overt hypothyroidism? 3) Are they symptomatic? 4) Does treatment with L-Thyroxine cure these symptoms,--if they exist? Summarizing the results of the literature one can give the following answers: 1) Elevated TSH with normal free T4 can but does not necessarily mean thyroid failure. 2) Patients with positive thyroid antibodies and especially with TSH levels above 10 mU/l are at high risk to develop overt hypothyroidism. 3) Typical symptoms (thyroid-specific, cardiovascular, neurological and psychiatric and finally alterations of risk factors for atherosclerosis) seem to occur in a greatly varying percentage of patients--some of the described symptoms are of questionable clinical importance. 4) Some of the symptoms, especially the cardiovascular, seem to be treatable by L-T4, whereas others like most of the changes in lipid metabolism can not be influenced by normalization of the TSH levels. In conclusion, screening for TSH and free T4 seems to be justified in elderly women, where the prevalence of the disease is approximately 20%. However, treatment of "symptoms" of subclinical hypothyroidism like elevated cholesterol levels or depression should be done only in patients with a TSH > 10 mU/l and there only with great caution in order to avoid unnecessary overdosage with the danger of eliciting atrial fibrillation.